The Chemistry of Pharaoh's Serpent with Calcium Gluconate

serpent with desert background and text: "Pharaoh's Serpent Decoded" - preview image

The “Pharaoh’s Serpent” chemical reaction involves the thermal decomposition of mercury (II) thiocyanate:1,2

2 Hg(SCN)2 (s) + 3 O2(g) 2 HgS(s) + C3N4(s) + 2 SO2(g) + CO2(g)       Equation 1

As the gaseous products permeate the solids formed during the reaction, a column of foam is created that looks like a snake. While this reaction is very interesting to observe, the toxicity associated with this experiment is enough to keep me from using it in the classroom.

However, I recently became aware of a similar “chemical serpent” that can be generated by the thermal decomposition of calcium gluconate:3,4

CaC12H22O14(s) 11 C(s) + 11 H2O(g) + CaCO3(s)             Equation 2

This reaction can be carried out very simply by igniting a fuel tablet (like those used for camping) and placing calcium gluconate tablets (sold as supplements) on top. Better yet, both reactants and products are completely benign. Because of this, I had to try the reaction out for myself. When doing so, I noticed quite a few interesting details (see Video 1).

Video 1: The Chemistry of Pharaoh's Serpent Explained!, Tommy Technetium YouTube Channel, February 19, 2024.

 

There is a LOT of chemistry involved in this experiment! Consider, for example, the chemical reaction that occurs as the fuel tablet burns. These tablets are made of hexamine (C6H12N4), which combusts according to:5

C6H12N4(s) + 9 O2(g) 6 CO2(g) + 2 N2(g) + 6 H2O(g)                  Equation 3

The decomposition of calcium gluconate (Equation 2) also displays a variety of interesting physicochemical phenomena. First, is the creation of the serpentine black foam, which fades to grey and white over time. Second, the foamy snake emits an orange light when it is black, but white light as its color fades to grey or white. I found a study published on the thermal decomposition of calcium gluconate and noted that all these features can be explained by the chemistry that occurs. As noted in the video, the carbon that is produced (Equation 2) is what gives the foam its black color. This carbon is also responsible for the orange light that is emitted, because carbon glows an incandescent orange when heated. The observed shift in the color of the foam from black to white can also explained by noting that the carbon is removed as it is converted into CO2 (Equation 4) leaving behind white CaCO3:

C(s) + O2(g) → CO2(g)        Equation 4

It is mostly gaseous water (Equation 2) that permeates the solid products to cause the foam, but I wonder if the CO2 produced (Equation 4) also contributes. The bright white light emitted from the foam results as CaCO3 is converted into CaO, which is also known as lime:

CaCO3(s) → CaO(s) + CO2(g)           Equation 5

And when lime is heated to high temperatures, it emits bright white light: It’s limelight!

Adding together Equations 3-5 we can write an overall reaction for the reaction between calcium gluconate and oxygen to form gaseous water, carbon dioxide, and calcium oxide:

CaC12H22O14(s) + 11 O2(g) → 12 CO2(g) + 11 H2O(g) + CaO(s)       Equation 6

I found it interesting to use various thermodynamic data (Table 1) to look at the reactions described in Equations 2,4, and 5 in more detail. Using the familiar equations:

           Equation 7

                     Equation 8

ΔG = ΔH - TΔS                                              Equation 9

the values of , , and  for reactions 3-5 displayed in Table 2 are obtained.

 

Table 1: Thermodynamic data of the species in Equations 2, 4, and 56-8

Compound

ΔHfo / kJ mol-1

So / J mol-1 K-1

CaC12H22O14 -3545 472
CaO(s) -635 38
CaCO3(s) -1207 93
CO2(g) -394 214
C(s, amorphous) 20 20
H2O(g) -242 189
O2(g) 0 205

 

Table 2: Thermodynamic values for Equations 2, 4, and 5 at T = 298.15 K

Equation

ΔHrxno / kJ mol-1

So / J mol-1 K-1

ΔGrxno / kJ mol-1

2 -104 +1920 -676
4 -374 -11 -371
5 +178 +159 +131

While the reaction described in Equation 2 is only slightly exothermic (-104 kJ mol-1), ΔG for this reaction is very negative. This is likely because the associated increase in entropy for this reaction is very positive (+1920 J mol-1 K-1). Perhaps some of the free energy from the reaction in Equation 2 provides energy to create the foam. It is interesting to note that the reaction described in Equation 5 is not spontaneous at normal temperatures. The temperature at which this reaction is expected to become spontaneous can be estimated by insertion of ΔH° and ΔS° for Equation 5 into Equation 8:

T ≈ ΔH/ΔS                  Equation 8

Upon doing so, a value of T = 1120 K is obtained. The energy from the combusting hexamine tablet certainly contributes to a higher temperature. It could also be that the large free energy release from Equations 2 and 4 could account for heating the foam to higher temperatures, allowing the CaO product to emit limelight!

This reaction has so many fantastic features in addition to the creation of the foamy serpent. It emits different colors of light and displays a subtle color change. The reaction is easy to set up, to carry out, and clean up. It connects to five different chemical reactions and a variety of topics in chemical thermodynamics. I highly recommend trying this experiment out for yourself. If you do, be sure to let me know what kinds of things you observe, and what kind of chemical topics you think about as you watch the reaction play out.

Happy Experimenting!

References:

  1. https://pubs.acs.org/doi/epdf/10.1021/ed017p268
  2. https://web.archive.org/web/20120201221503/http://chemistry.about.com/od/fireworksprojects/a/pharaohs-snakes.htm
  3. https://melscience.com/US-en/articles/gluconate-snake-experiment/
  4. Labuschagne; F. J. W. J.; Focke, W. W. Metal catalysed intumescence: characterisation of the thermal decomposition of calcium gluconate monohydrate J. Mater. Sci. 38, 2003 1249–1254.
  5. Merritt, J. R.; Herington, L.; Jones, S. B.; Sayed, Y. Analysis of Hexamine Combustion Am. Ind. Hyg. Assoc. J. 52, 1991, 30-33.
  6. Di, Y.-Y.; Zhang, G.-C.; Lui, Y.-P.; Kong, Y.-X.; Zhou, C.-S. Crystal structure and thermodynamic properties of the coordination compound calcium D-gluconate Ca[D-C6H11)7]2(s) J. Molec. Sruct 1225, 2021, 1-10.
  7. Eisermann, W.; Johnson, P.; Conger, W. L. Estimating Thermodynamic Properties of Coal, Char, Tar, and Ash. Fuel Processing Technology, 3, 1980, 39-53.
  8. Ebbing, D. D.; Gammon, S. D. General Chemistry, 11th ed.; 2017, Cengage Learning.

Note: Both the camp fuel pellets and the calcium gluconate supplement tablets can easily be found online and in stores. One online option for the fuel pellets is Coghlan's Fuel Stove Tablets on Amazon. An option for the calcium gluconate tablets is the Nature Made brand on Amazon.

 

Safety

Safety: Video Demonstration

Demonstration videos presented here are not meant as tools to teach chemical demonstration techniques. They are meant as a tool for classroom use. The demonstrations may present safety hazards or show phenomena that are difficult for an entire class to observe in a live demonstration.

Those performing the demonstrations shown in this video have been trained and adhere to best safety practices.

Anyone thinking about performing a chemistry demonstration should first read and then adhere to the ACS Safety Guidelines for Chemical Demonstrations (2016) These guidelines are also available at ChemEd X.

General Safety

For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).  

For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.

Other Safety resources

RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies

 

NGSS

Students who demonstrate understanding can use a model to describe how variations in the flow of energy into and out of Earth’s systems result in changes in climate.

More information about all DCI for HS-ESS2 can be found https://www.nextgenscience.org/dci-arrangement/hs-ess2-earths-systems and further resources at https://www.nextgenscience.org.

Summary:

Students who demonstrate understanding can use a model to describe how variations in the flow of energy into and out of Earth’s systems result in changes in climate.

Assessment Boundary:

Assessment of the results of changes in climate is limited to changes in surface temperatures, precipitation patterns, glacial ice volumes, sea levels, and biosphere distribution.

Clarification:

Examples of the causes of climate change differ by timescale, over 1-10 years: large volcanic eruption, ocean circulation; 10-100s of years: changes in human activity, ocean circulation, solar output; 10-100s of thousands of years: changes to Earth's orbit and the orientation of its axis; and 10-100s of millions of years: long-term changes in atmospheric composition.

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.

Summary:

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

Assessment Boundary:

Assessment is limited to chemical reactions involving main group elements and combustion reactions.

Clarification:

Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.

Students who demonstrate understanding can develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.

*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.

Summary:

Students who demonstrate understanding can develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.

Assessment Boundary:

Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.

Clarification:

Emphasis is on the idea that a chemical reaction is a system that affects the energy change. Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.

Students who demonstrate understanding can create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.

*More information about all DCI for HS-PS3 can be found at https://www.nextgenscience.org/topic-arrangement/hsenergy

Summary:

Students who demonstrate understanding can create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.

Assessment Boundary:

Assessment is limited to basic algebraic expressions or computations; to systems of two or three components; and to thermal energy, kinetic energy, and/or the energies in gravitational, magnetic, or electric fields.

Clarification:

Emphasis is on explaining the meaning of mathematical expressions used in the model. 

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Comments 1

Nick Thomas's picture
Nick Thomas | Wed, 04/10/2024 - 14:56

Outstanding demonstration and explanation, Tom. I’ll just add that the Amazon link goes to K gluconate pills. Since they were cheap ($4), I tried them. I expected that would make some carbon, which they did – but not a lot and more slowly – but no white glow since no CaO would be formed. Ca gluconate tablets (at least on Amazon) were about $20. But a super cool demo. Thanks for sharing.